Quick
The safety factor may also be called the factor of safety. The safety factor may be abbreviated by F.S., SF, or n. The safety factor is a ratio of the ultimate capacity to the allowable capacity. Ultimate can be thought of as the maximum value that can be tolerated by a part without failure. Allowable can be though of as the maximum value to be experienced by the part for the duration of its working life.
Equations
and also
Implications
Understanding the definitions of ultimate and allowable, it can be seen that as the safety factor decreases, the possibility of failure increases. Conversely, as the safety factor increases, the part becomes less economical and functional. Therefore, the safety factor must be chosen appropriately to obtain the correct balance between reliability, economy, and functionality for a given application.
Deciding upon a reasonable safety factor is dependent upon various criteria such as the following:
- Variations of the properties of the part under consideration. For example, the composition, strength, and dimensions of the part are some properties that are subject to minor variations during manufacture. Also, material properties may be altered and residual stresses introduced via heating or deformation that may occur during manufacture, storage, transportation, or construction.
- The number of loadings that may be expected during the life of the part. For most materials the ultimate strength decreases as the number of load applications increases. If this aspect, called fatigue, is ignored, sudden failure may occur.
- The type of loadings that are planned for the part, or that may occur in the future. Very few loadings are known with complete accuracy—most design loadings are engineering estimates. In addition, future alterations or changes in usage may introduce changes in the actual loading. Larger factors of safety are required for dynamic, cyclic, or impulsive loadings.
- The type of failure that may occur. Most buckling or stability failures are sudden, whether the material is brittle or not. When the possibility of sudden failure exists, a larger factor of safety should be used than when failure is preceded by obvious warning signs.
- Uncertainty due to methods of analysis. All design methods are based on certain simplifying assumptions which result in calculated stresses being approximations of actual stresses.
- Deterioration that may occur in the future because of poor maintenance or because of unpreventable natural causes. A larger factor of safety is necessary in locations where conditions such as corrosion and decay are difficult to control or discover.
- The importance of a given member to the integrity of the whole structure. Bracing and secondary members may in many cases be designed with a factor of safety lower than that used for primary members.
- Risk to life and or property. Where a failure would produce no risk to life and only minimal risk to property, the use of a smaller factor of safety may be considered.
The selection of a reasonable safety factor requires a good knowledge of design, a thorough understanding of the strength of the material, and application engineering experience.
Related
▪ L - Design Factor and Safety Factor